1. Field of the Invention
The present invention relates to an eddy current testing device for evaluating a change in characteristics of a target object to be inspected, such as a crack and a change in a property of a material of the target object, and more particularly to an eddy current testing device that uses an eddy current probe having a mutual induction type standard comparison scheme to evaluate a change in characteristics of a target object to be inspected.
2. Description of the Related Art
A method for inspecting a metal body (which is a target object) using an eddy current is to supply an alternating current (exciting current) to a coil provided in an eddy current probe, cause the coil to generate an alternating magnetic flux, cause the eddy current probe to approach the metal body and generate an eddy current, and detect a signal indicating turbulence of the eddy current. The eddy current varies depending on conductivity, permeability and the like of the target object. It is therefore possible to evaluate a change in characteristics of the target object (such as a crack of the target object, a change in a property of a material of the target object, and the like) to be inspected under the condition that the eddy current probe is not in contact with the target object. Specifically, the eddy current probe is used to detect a standard detection signal from a well-conditioned standard sample piece in advance, and a difference between a signal detected from the target object and the standard detection signal is displayed on a screen. An inspector can confirm a change in the characteristics of the target object by viewing the display screen.
A Lissajous pattern showing amplitude and a phase angle of the detected signal is known as a method for displaying the detected signal. Specifically, the detected signal (voltage) is converted into an X component voltage Vx and a Y component voltage Vy. The X component voltage Vx has the same phase as that of the standard signal. The Y component voltage Vy is represented in a direction perpendicular to a direction in which the X component voltage Vx is represented. After the conversion, the X component voltage Vx is plotted along an X axis, and the Y component voltage Vy is plotted along a Y axis, to represent the amplitude |V| and phase angle θ of the detected signal (refer to the following expressions (1) and (2)). The inspector can determine whether or not a flaw signal is detected based on the shape of a Lissajous pattern.|V|=(Vx2+Vy2)−1/2  (1)θ=tan−1(Vy/Vx)  (2)
In recent years, a scanning mechanism for moving an eddy current probe along a target object has been used, or a multi-coil probe having coils regularly arranged has been used as an eddy current probe, to inspect a wide area of the target object. In those cases, since the position of a portion of the target object, from which a flaw signal is detected, cannot be confirmed based on a Lissajous pattern, a display method (C scope) is used to indicate the amplitude of the detected signal by means of grayscale shading in two dimensional coordinates in which the position of the portion of the target object is plotted along a coordinate axis (refer to, for example, Non-Patent Document 1 (Nishimizu, etc., “Development of Flexible Multi Eddy Current Testing Sensor”, The 8th Symposium on Nondestructive Surface Testing (2005), pp. 139-142).